1. Field of the Invention
The invention relates to a process for exposing the peripheral area of a wafer, a process which is used for removal of an unnecessary resist on a wafer in a development process, and a device for executing the process. The invention relates especially to a process for exposing the peripheral area of a wafer, in which some of the peripheral area of the wafer is exposed in a step shape and the remainder is exposed in a ring shape, and a device for executing the process.
2. Description of the Related Art
In the manufacture of a semiconductor device, for example, an IC, LSI or the like, a photoresist (hereafter called a resist) is applied to the surface of a semiconductor wafer (hereafter called a wafer), such as a silicon wafer or the like. In this manner, a circuit pattern is formed.
The peripheral area of a wafer, however, is only rarely used as an area in which the pattern is formed. In the case in which the resist is a positive resist, the peripheral area is not exposed, and as a result, the resist remains in the peripheral area even after development. This resist which has remained in the peripheral area causes impurities on peripheral devices and, as a result, impurities on the wafer surface and thus a reduction in yield due to loosening or the like during transportation and holding of the wafer.
Therefore, a proposal has been advanced for removing the unnecessary resist in the peripheral area of the wafer. To do this, a process for exposing the peripheral area of the wafer is carried out in which, regardless of the process of exposure in the area in which the pattern is formed, the unnecessary resist in the peripheral area is removed.
Conventionally, the processes described below have been used as processes for exposure of the above described peripheral area of a wafer:
(1) While one exit end, from which exposure light emerges, is moved parallel to the wafer surface in orthogonally intersecting directions, the peripheral area of the wafer is irradiated with the exposure light. Alternatively, instead of the exit end, the wafer may be moved. The process exposes the peripheral area of the wafer in a step shape, as is illustrated in FIG. 17(a). This exposure process is hereinafter called step-shaped exposure.
(2) While rotating a wafer on which the resist has been applied, the peripheral area of the wafer is irradiated with exposure light by which the entire periphery, or part, of the wafer is exposed in a ring shape, as is illustrated in FIG. 17(b). This exposure process is hereinafter called ring-shaped exposure.
The step-shaped exposure described above in (1) is often used for exposure of a peripheral area in the case in which the circuit pattern on the wafer is gradually exposed using a reduction projection exposure device of the movement type (stepper). In the above described gradual exposure, the periphery of the area to be exposed, in which the circuit pattern is formed, is formed in a step shape which changes according to the exposure pattern because several circuit patterns, which each correspond to one chip at a time, are formed on the wafer.
Therefore, in the peripheral area of the wafer, a step-shaped part is formed which is not yet exposed and some of the unnecessary resist has a step shape. This unnecessary resist causes the above described decrease of yield due to loosening or the like. The peripheral area of the wafer is therefore exposed in a step-shape by the process described above in (1), by which formation of the not yet exposed part on the wafer is prevented.
Recently, there has been a greater demand for exposure of the peripheral area of the wafer in which the above described step-shaped and ring-shaped exposures are used at the same time. In this process, some of the peripheral area of the wafer is exposed in a step-shape and the remainder is exposed in a ring-shape, as is illustrated in FIG. 17(c). To satisfy this demand, conventionally two exposure devices are used, i.e., one exposure device for step-shaped exposure of the peripheral area of the wafer, as disclosed in, for example, Japanese patent disclosure document HEI 4-291938, and one exposure device for ring-shaped exposure of the peripheral area of the wafer, as disclosed in, for example, Japanese patent disclosure document HEI 2-1114 (U.S. Pat. No. 4,899,195). Thus, two processes for exposure of the peripheral area of a wafer are executed. One of the devices is used to expose some of the peripheral area of the wafer, for example, in a step-shape, and then the wafer is transported to the other of the devices which exposes the remainder of the peripheral area of the wafer in a ring-shape.
The above described prior art, however, has the following disadvantages:
(1) It is necessary to make available two exposure devices, that is, the exposure device for step-shaped exposure of the peripheral area of a wafer, and the exposure device for ring-shaped exposure of the peripheral area, which increases the floor surface occupied by the exposure devices in a clean room.
(2) It is necessary to place the wafer on one of the exposure devices, to determine the placed state of the wafer, execute positioning and exposure, transport the wafer to the other exposure device, and then, again, to determine the placed state of the wafer, and to execute positioning and exposure. For this reason several working processes are necessary.
(3) To execute the above described exposures at the same station, use of an exposure device for step-shaped exposure of the peripheral area of a wafer is conceivable. However, in this case, the following problem occurs:
As is known, for example, from Japanese patent disclosure document HEI 4-291938, in the above described exposure device, position data of the exit end, from which the exposure light emerges, is computed as two-dimensional rectangular coordinates. Position control of the above described exit end is then performed. For ring-shaped exposure of the peripheral area of a wafer, it is therefore necessary to increase the resolution of the above described rectangular coordinates. As a result, the amount of data is increased and a memory with greater capacity is necessitated. Furthermore, if a microprocessor unit (MPU) which has a high working speed is not used, the throughput decreases compared to the exposure device which is known, for example, from Japanese patent disclosure document HEI 2-1114. Thus, the device becomes expensive. If the resolution of the above described data is reduced, there is the tendency that the above described problem is solved. However, instead, on the boundary of the area to be exposed in a ring-shape, there is an extremely small step-shape and loosening of the resist proceeds from this boundary area.